The present invention relates to automatic frequency control (AFC) systems for use in television receivers and, more particularly, to circuitry for quickly charging and discharging the AFC filter capacitor of the AFC system.
The above referenced patent application discloses a video demodulator system for use in a television receiver wherein the demodulator system comprises an internal phase locked loop (PLL) in which a error signal is generated for driving the AFC loop of the receiver. As disclosed, the error signal is generated from the phase detector portion of the PLL and is utilized to provide a control voltage to adjust the local oscillator of the AFC loop in order to lock the video demodulator system to the IF composite television signal. The AFC voltage is derived by charging or discharging an AFC capacitor accordingly. This AFC voltage tunes the local oscillator to pull in and lock to the television composite signal.
Ideally, it is desired to have a large AFC filter to reduce or eliminate amplitude and phase modulation errors on the AFC signal(which has the effect of reducing the loop bandwidth). However, in contrast, it is desirable for the loop bandwidth of the AFC system to be as wide as possible in order to reduce AFC lock up time.
Hence, a need exist for an AFC system having means for rapidly charging and discharging the AFC filter capacitor when the PLL VCO is not at its normal operating frequency which enables a larger than normal filter capacitor to be realized thus reducing AFC loop bandwidth without increasing AFC capture time.